1. Field
The present application generally relates to optical metrology, and, more particularly, to determining transmittance of a photomask using optical metrology.
2. Description of the Related Art
Semiconductor devices/circuits are formed on semiconductor wafers by depositing and patterning layers of materials. In general, the features of the devices/circuits are formed onto the layers of deposited materials using a lithography process.
In a typical lithography process, the features of the devices/circuits are laid out, one layer at a time, on a series of photomasks. The layout of the features of the devices/circuits on the photomasks are transferred, one photomask at a time, onto the deposited layers of materials. A single photomask typically includes the layout for one layer of one or more chips throughout the wafer.
For example, with reference to FIG. 1, a photoresist layer 100 is deposited on wafer 102. Photomask 104 is positioned above photoresist layer 100. Photomask 104 includes portions 106 that block light and portions 108 that transmit light. Portions 106 of photomask 104 that block light can be patterned to have the same shape as the features that are to be formed on photoresist layer 100. These types of photomasks are generally referred to as “light field” photomasks. Alternatively, portions 108 of photomask 104 that transmit light can be patterned to have the same shape as the features that are to be formed on photoresist layer 100. These types of photomasks are generally referred to as “dark field” photomasks. For the sake of convenience and clarity, photomask 104 is depicted and described as being a “light field” photomask.
After photomask 104 is aligned, photomask 104 and portions of photoresist layer 100 are exposed to light. As depicted in FIG. 1, only certain portions of photoresist layer 100 are exposed to the light, i.e., the portions under portions 108 of photomask 104 that transmit light. As also depicted in FIG. 1, a lens 110 can be disposed between photomask 104 and photoresist layer 100 to focus the transmitted light.
Photoresist layer 100 has the material characteristic that its solubility is responsive to exposure to light. More particularly, some photoresist change from a soluble to an insoluble condition when exposed to light. These types of photoresist are generally known as “negatively acting” resist. In contrast, some photoresist change from an insoluble to a soluble condition when exposed to light. These types of photoresist are generally known as “positively acting” resist. For the sake of convenience and clarity, assume that photoresist layer 100 is a “positively acting” resist. As such, when photoresist layer 100 is exposed to an appropriate chemical solvent (i.e., a developer), the portions of photoresist layer 100 that were exposed to the light are dissolved.
When making photomask 104, it is desirable to determine the transmittance of photomask 104. For the purpose of this application, the term “transmittance” refers to a measurement of the amount of light transmitted through photomask 104 (i.e., an amplitude) and/or the associated phase shift that results in the light after being transmitted through photomask 104 (i.e., a phase shift). Stated another way, the term “transmittance” can be used to refer to only the amplitude without the phase shift, only the phase shift without the amplitude, or both the amplitude and the phase shift.
Conventionally, transmittance of photomask 104 is determined by actually measuring the amount of light transmitted through photomask 104 and/or the phase shift. This process, however, requires specialized equipment, which can be costly and time consuming to use.